So-called resonant converters have a resonant circuit, which can be a series or parallel or series-parallel resonant circuit. When configuring converters, one aim is to keep losses low. For example, resonant converters which comprise an LLC series-parallel resonant circuit having two inductances and one capacitance are well-known. Such converters have the advantage that energy-efficient operation with relatively low switching losses is possible.
Resonant LLC converters are well known for use within LED drivers. The converters can be configured or operated as a constant current source or a constant voltage source. A constant current source can be used to drive an LED arrangement directly, thus enabling a single stage driver. Constant voltage sources can be used, for example, for LED modules which have further driver electronics in order to ensure a corresponding power supply to the LEDs with a predetermined current from the output voltage provided by the constant voltage source.
The LLC converter comprises a switching arrangement (which together with the gate driving arrangement is generally referred to as the inverter) for controlling the conversion operation, and the switching is controlled using feedback or feedforward control, in order to generate the required output.
Another function implemented within a power converter which is supplied with mains (or other AC) power is power factor correction (PFC). The power factor of an AC electrical power system is defined as the ratio of the real power flowing to the load to the apparent power in the circuit. A power factor of less than one means that the voltage and current waveforms are not in phase, reducing the instantaneous product of the two waveforms. The real power is the capacity of the circuit for performing work in a particular time. The apparent power is the product of the current and voltage of the circuit. Due to energy stored in the load and returned to the source, or due to a non-linear load that distorts the wave shape of the current drawn from the source, the apparent power will be greater than the real power.
If a power supply is operating at a low power factor, a load will draw more current for the same amount of useful power transferred than for a higher power factor.
The power factor can be increased using power factor correction. For linear loads, this may involve the use of a passive network of capacitors or inductors. Non-linear loads typically require active power factor correction to counteract the distortion and raise the power factor. The power factor correction brings the power factor of the AC power circuit closer to 1 by supplying reactive power of opposite sign, adding capacitors or inductors that act to cancel the inductive or capacitive effects of the load.
Active PFC makes use of power electronics to change the waveform of the current drawn by a load to improve the power factor. Active PFC circuits may for example be based on buck, boost or buck-boost switched mode converter topologies. Active power factor correction can be single-stage or multi-stage.
In the case of a switched mode power supply, a PFC boost converter is for example inserted between the bridge rectifier and the mains storage capacitor. The boost converter attempts to maintain a constant DC bus voltage on its output while drawing a current that is always in phase with and at the same frequency as the line voltage. Another switched-mode converter inside the power supply produces the desired output voltage or current from the DC bus.
Due to their very wide input voltage range, many power supplies with active PFC can automatically adjust to operate on AC power for example from about 110 V to 277V.
Power factor correction may be implemented in a dedicated power factor correction circuit (called a pre-regulator), for example placed between the (mains) power supply and the switch mode power converter which then drives the load. This forms a dual stage system, and this is the typical configuration for high power LED applications (for example more than 25 W). The power factor correction may instead be integrated into the switch mode power converter, which then forms a single stage system.
In this case, there is a single resonant tank and switching arrangement, which then implements both power factor correction as well as control of the conversion ratio between the input and output in order to maintain the desired output (current in the case of an LED driver) delivered to the load.
LLC DC/DC converters are either operated at a DC supply voltage (e.g. 48V in telecommunications or data center applications), or they are used as the second stage of a mains power supply or two stage LED driver, in which the front end stage (the power factor correction pre-regulator) provides the power factor correction and also generates a stabilized bus voltage that forms the DC input voltage for the LLC.
An example of a resonant AC/DC converter is shown in FIG. 1.
The circuit comprises a DC input terminal 2 (labeled B in FIG. 1 and all other figures) which connects to a half-bridge having a first power switch 28 and a second power switch 30. The first switch and the second switch can be identical, and the half-bridge may for example operated at a symmetrical 50% duty cycle. These switches can be in the form of field-effect transistors.
A resonant tank circuit 25 is connected to a switch node, labeled X in FIG. 1 and all other figures between the two switches 28, 30.
Each switch has its timing of operation controlled by its gate voltage. For this purpose, there is a control block 31 (including a low voltage supply). The block 31 receives a control signal CTRL for controlling the gate voltages and a supply voltage SUP. Feedback (not shown) is used to determine the timing of the control of the switches 28, 30. The output of the resonant tank circuit 25 connects to a rectifier 32 and then to the load, in parallel with a smoothing capacitor CDC.
During operation of the converter, the controller 31 controls the switches, at a particular frequency and in complementary manner.
FIG. 2 shows one more detailed example of the circuit of FIG. 1.
In this example, the resonant tank 25 is in the form of an LLC resonant circuit, and it may be used to form a PFC stage. The circuit may thus be used as a PFC pre-regulator by having a controlled output voltage. It could also be used as a single stage LED driver by having a controlled output current.
The circuit comprises a mains input 10 which is followed by a rectifier bridge 12 having a high frequency filter capacitor 14 at the output. This generates the supply for the input terminal 2 (node B) of FIG. 1.
This example shows a converter with an isolated output. For this purpose, the converter comprises a primary-side circuit 16 and a secondary side 18. There is electrical isolation between the primary-side circuit 16 and the secondary side 18. A transformer comprising a primary coil 20 and a secondary coil 22 is provided for the isolation. The transformer has a magnetizing inductance 20 which also acts as one of the inductances of the series LLC resonant circuit. The LLC resonant circuit 25 has a second inductance 24, and a capacitance (formed as two capacitors 26 and 27 in this example).
In an LLC circuit, the inductances and capacitor may be in any series order. The inductor may comprise discrete components or it may be implemented as leakage inductances of the transformer.
The primary-side circuit 16 comprises the half-bridge 28, 30 and the resonant tank circuit 25.
The control block 31 is shown schematically as including two voltage sources.
The secondary side 18 has the rectifier 32 which is connected downstream of the secondary coil 22 and which can be formed, for example, by a first diode arrangement of diodes 32a and 32b and a second diode arrangement of diodes 34a and 34b. 
FIG. 2 shows a full-bridge rectifier and a single secondary coil which couples at its ends to the rectifier circuit. The low frequency (e.g. 100 Hz) storage capacitor CDC is connected between the outputs of the rectifier. The LED load or other output stage is represented in this figure by a resistor. It comprises an LED or a plurality of LEDs.
The circuit shown in FIG. 2 is thus an AC/DC PFC converter, comprising an AC input 10, a rectifier 12, a half bridge inverter comprising a high side switch (the first power switch 28) and a low side switch (the second power switch 30), wherein an output is defined from a switch node X between the switches. The self-oscillating LLC circuit 20,24,26,27 is coupled to the output.
FIG. 3 shows an alternative LLC half bridge topology, as a modification to FIG. 2 (and showing DC/DC conversion) in which the secondary coil 22 has a center tap and the full wave rectifier 32 is then implemented by two diodes. The LLC capacitor is also shown as a single component 35.
The half bridge converter shown above may be used in a AC/DC (single stage) PFC converter, or in a DC/DC converter, or in an AC/DC converter without implementing power factor correction. In the case of a DC/DC converter, the rectifier bridge 12 and filter capacitor 14 are simply omitted as in FIGS. 1 and 3. The half bridge converter may also be used in a DC/AC converter, i.e. a resonant half bridge inverter. The resonant tank circuit 25 may also be of other types, and the invention is not limited to LLC circuits.
In the case of DC/AC conversion, a load is connected to the output of the resonant tank circuit whereas in case of DC/DC or AC/DC conversion the load is connected via the active or passive rectifier network to the resonant tank circuit.
Half bridge resonant converters are used already in many applications like DC/AC converters for lighting applications, e.g. low- and high-pressure discharge lamp circuits, and DC/DC converters, e.g. DC power supplies and LED drivers.
The control block 31 drives the two power switches 28, 30 to conduct in an alternating sequence on and off, with a small non-conduction phase (dead time) used to avoid cross conduction of the power switches. A high gate drive signal turns on one switch and turns off the other switch and a low gate drive signal turns off the one switch and turns on the other switch. The advantage of using a resonant half bridge converter is that the current flowing into the switch node X has a phase lag, with respect to the switch node voltage Vx, and can serve to discharge the (parasitic) output capacitance of the switch before it will be switched-on.
This method is referred to as Zero Voltage Switching (ZVS) and implies zero switching losses due to the parasitic output capacitance. If the output current is not large enough or even zero and further depending on the operation conditions (in terms of the half bridge, output, and resonant capacitor voltage), discharging of the parasitic output capacitance will be partly or even completely achieved by the power switch which results in hard switching. This results in switching losses which depend on the switching frequency, the parasitic output capacitance of the switch and the voltage across the parasitic capacitance at switch-on. In order to reduce the switching losses, Valley Switching (VS) can be applied which causes a switch to switch-on at the minimum voltage across it. Valley switching can be implemented by means of an end-of-slope detection mechanism. Zero voltage switching is a special case of valley switching where the voltage is minimal and zero.
In order to avoid critical timing of the switch turn-on, a diode can be placed in anti-parallel to the power switch 28, 30 if a bipolar junction transistor is used. This anti-parallel diode may be omitted for a MOSFET because it already has a body diode inside. The anti-parallel diode will start conducting if the switch is not switched on immediately after discharging of the voltage across the switch has occurred, and then the switch can take over a bit later when it is eventually turned on.
Zero voltage switching ensures that the voltage across a switch is zero before it will be switched on and as such eliminates switching losses which makes high frequency (HF) operation possible. HF operation enables a reduction in the size of capacitive and inductive components used in the resonant tank circuit which makes smaller and cheaper designs possible.
In these circuits, the first power switch 28 connected to the rectified mains (or other DC input) needs a drive signal which should be close to the switch node voltage Vx which can range from ground up to the high rectified mains voltage (or other DC voltage) at terminal 2 for switching on and off. This means that a level shifter function is needed.
FIG. 4 shows a driver transformer for this purpose. There are two secondary coils 40, 42 each connected across the source and drain of a respective one of the power switches 28, 30. The secondary coil 40 sets the gate voltage of the first power switch 28 relative to the switch node X and the secondary coil 42 sets the gate voltage of the second power switch 30 relative to ground. The secondary coils have opposite polarity to provide the complementary switching.
FIG. 5 shows a high voltage level shifting integrated circuit 50 having a level shifting unit 52 and gate driver circuits 54, 56 for the first and second power switches 28, 30.
By way of example, it may be desired to implement switching frequencies as high or even higher than 1 MHz and with a maximum rectified mains voltage of 375V. This voltage level should be able to be raised to at least 500V whilst still preventing damage of the switches and drive circuits during mains surges.
The two level shift implementations shown have drawbacks.
A transformer level shifter can be used for both low frequency and high frequency operation and an isolation voltage of 500V can indeed be achieved. However, it draws four times more power than needed to supply the gate charge of the power switch and the unavoidable leakage inductance in the transformer causes ringing. In the case of low frequency applications, the extra dissipation might be not a problem but for high frequency applications, the additional power dissipation will be an issue. Additionally, the ringing suppression measures which may be required cause severe turn on/off delays which might be not acceptable in high frequency operations.
The high voltage IC level shifter is currently only available for low frequency operation, not higher than about 1 MHz.
This invention relates to an improvement to the system for generating and applying the control signals to the power switches of the half bridge converter to address the issues explained above.